This invention relates generally to thin-film transistor (TFT) processes and fabrication, and more particularly, to a TFT polycrystalline film, and method of using transition metals, such as nickel, to induce the crystallizing of an amorphous film.
The demand for smaller electronic consumer products with higher resolution displays, spurs continued research and development in the area of liquid crystal displays (LCDs). The size of LCDs can be decreased by incorporating the large scale integration (LSI) and very large scale integration (VLSI) driver circuits, presently on the periphery of LCDs, into the LCD itself. The elimination of externally located driving circuits and transistors will reduce product size, process complexity, a number of process steps, and ultimately the price of the product in which the LCD is mounted.
The primary component of the LCD, and the component that must be enhanced for further LCD improvements to occur, is the thin-film transistor (TFT). TFTs are typically fabricated on a transparent substrate such as quartz, glass, or even plastic. TFTs are almost exclusively used as switches to allow the various pixels of the LCD to be charged in response to the driver circuits. TFT performance will be improved, and driver circuit functions incorporated into TFTs, by increasing the electron mobility in the TFT devices. Increasing the electron mobility of a transistor results in a transistor having faster switching speeds. Improved TFTs having increased electron mobility yield smaller LCD screens, lower power consumption, and faster transistor response times. Further LCD resolution enhancements will require that the TFTs mounted on the transparent substrates have electron mobility characteristics rivaling IC driver circuits currently mounted along the edges of the screen. That is, display and driver TFT located across the entire display must operate at substantially the same level of performance.
The carrier mobility of typical thin-film transistors, with active areas formed from amorphous film, is poor, on the order of 0.1 to 0.2 cm.sup.2 /Vs. Carrier mobility is improved by using crystallized silicon. Single crystal silicon transistors, which are usually used in TFT driver circuits, have electron mobilities on the order of 500 to 700 cm.sup.2 /Vs. Polycrystalline silicon transistor performance is between the two extremes, having mobilities on the order of 10 to 400 cm.sup.2 /Vs. Thin-film transistors having mobilities greater than 100 cm.sup.2 /Vs would probably be useful in replacing LCD periphery mounted driver circuitry. However, it has been difficult to produce polycrystalline TFTs with electron mobilities of even 40 to 50 cm.sup.2 /Vs.
Single crystal silicon films, for use with LCDs, are difficult to fabricate when adhered to relatively fragile transparent substrates. A quartz substrate is able to withstand high process temperatures, but it is expensive. Glass is inexpensive, but is easily deformed when exposed to temperatures above 600.degree. C. for substantial lengths of time. Even the fabrication of polycrystalline silicon transistors has been very difficult due to the necessity of using low temperature crystalline processes when glass is involved. Current polycrystalization processes typically require annealing times of approximately 24 hours, at 600.degree. C., to produce TFTs having a mobility of approximately 30-50 cm.sup.2 /Vs. These processes are not especially cost effective due to the long process times, and the TFTs produces are not suitable for LCD driver circuits.
The direct deposition of amorphous silicon film is probably the cheapest method of fabricating TFTs. Typically, the transparent substrate is mounted on a heated susceptor. The transparent substrate is exposed to gases which include elements of silicon and hydrogen. The gases decompose to leave solid phased silicon on the substrate. In a plasma-enhanced chemical vapor deposition (PECVD) system, the decomposition of source gases is assisted with the use of radio frequency (RF) energy. A low-pressure (LPCVD), or ultra-high vacuum (UHV-CVD), system pyrolytically decomposes the source gases at low pressures. In a photo-CVD system the decomposition of source gases is assisted with photon energy. In a high-density plasma CVD system high-density plasma sources, such as inductively coupled plasma and helicon sources, are used. In a hot wire CVD system the production of activated hydrogen atoms leads to the decomposition of the source gases. However, TFTs made from direct deposition have poor performance characteristics, with mobilities on the order of 1 to 10 cm.sup.2 /Vs.
Solid phase crystallization (SPC) is a popular method of crystallizing silicon. In this process, amorphous silicon is exposed to heat approaching 600.degree. C. for a period of at least several hours. Typically, large batches of LCD substrates are processed in a furnace having a resistive heater heat source. TFTs made from this crystallization process are more expensive than those made from direct deposition, but have mobilities on the order of 50 cm.sup.2 /Vs. A rapid thermal anneal (RTA) uses a higher temperature but for very short durations of time. Typically, the substrate is subjected to temperatures approaching 700 or 800.degree. C. during the RTA, however, the annealing process occurs relatively quickly, in minutes or seconds. Glass substrates remain unharmed due to the short exposure time. Because the process is so rapid, it is economical to process the substrates serially. Single substrates can also be brought up to annealing temperatures faster than large batches of substrates. A tungsten-halogen, or Xe Arc, heat lamp is often used as the RTA heat source.
An excimer laser crystallization (ELC) process has also been used with some success in annealing amorphous silicon. The laser allows areas of the amorphous film to be exposed to very high temperatures for very short periods of time. Theoretically, this offers the possibility of annealing the amorphous silicon at its optimum temperature without degrading the transparent substrate upon which it is mounted. However, use of this method has been limited by the lack of control over some of the process steps. Typically, the aperture size of the laser is relatively small. The aperture size, power of the laser, and the thickness of the film may require multiple laser passes, or shots, to finally anneal the silicon. Since it is difficult to precisely control the laser, the multiple shots introduce non-uniformities into the annealing process. Further, the wafers must be annealed serially, instead of in a furnace in batches. Although mobilities of over 100 cm.sup.2 /Vs are obtainable, TFTs made by this method are significantly more expensive than those made by direct deposition or SPC.
Also under investigation is the use of metal, such as aluminum, indium tin oxide, and transition metals such as nickel, cobalt, and palladium to encourage the crystallization of silicon. Nickel seems especially promising, as the lattice constant of nickel disilicide is similar to silicon. In general, nickel has been used to reduce the annealing temperature typically required in a conventional solid phase crystallization (SPC) from approximately 600.degree. C. to a temperature in the range between approximately 500 to 550.degree. C., so that the LCD substrates are less susceptible to shrinkage. The use of nickel also significantly shortens the annealing process times. TFTs made through this process cost about the same as those made with the SPC method, however, the mobilities of metal-induced TFTs can approach 100 cm.sup.2 /Vs. Liu et al., U.S. Pat. No. 5,147,826, discloses the deposition of a nickel film on amorphous silicon so that the annealing temperature can be reduced to approximately 550 to 650.degree. C. Fornash et al., U.S. Pat. No. 5,275,851 disclose a similar process. However, neither method produces polycrystalline silicon TFTs with very high electron mobility.
An improvement to the SPC or laser annealing process is presented in co-pending U.S. Pat. Ser. No. 08/812,580, filed Mar. 7, 1997, entitled "Polycrystalline Silicon from the Crystallization of Microcrystalline Silicon and Method for Same", invented by Tolis Voutsas, now U.S. Pat. No. 5,827,773, which is assigned to the same assignees as the instant application. This patent application discloses the use of amorphous film with embedded microcrystallites to produce polycrystalline silicon. The polycrystalline silicon has a more uniform distribution of crystal structures, and larger crystal grains. However, the invention does not address the subject of increasing the quality, and reducing the costs, of metal-induced crystallized film.
The process of heating amorphous silicon to form crystallized silicon is not entirely understood, and research on the subject continues. Variations in temperature, film thickness, the degree to which the amorphous matter melts, impurities in the film, and a range of other factors influence the annealing of amorphous silicon. Generally, large grains of crystallization, or crystallization able to support high carrier mobilities, occur in a polycrystalline film at a specific temperature near the melting point. Temperatures below this preferred temperature do not melt the amorphous silicon enough to form large grain areas, or to form uniformly crystallized film. Temperatures above the preferred temperature rapidly lead to bulk nucleation. The bulk nucleation of amorphous matter results in the spontaneous crystallization of an amorphous film into relatively small grain sizes so that the electron mobility is relatively poor.
It would be advantageous if a method were found of annealing amorphous silicon to form polycrystalline TFT transistors on glass substrates with electron mobilities exceeding 100 cm.sup.2 /Vs.
It would be advantageous if a method were found of improving the, low cost, metal-induced crystallization to form polycrystalline TFT transistors having electron mobilities exceeding 100 cm.sup.2 /Vs. Further, it would be advantageous if, simultaneously, an RTA process could be incorporated to reduce annealing process times, and so, further reduce annealing costs.
Accordingly, in forming thin-film transistors with high electron mobility, a method for crystallizing an amorphous film is provided, comprising the steps of:
a) depositing a layer of the amorphous film having a first thickness; PA1 b) depositing a layer of transition metal film having a second thickness overlying, and in contact with, the amorphous film; PA1 c) annealing the films deposited in steps a) and b) so that a portion of the amorphous film underlying the transition metal is consumed to form a transition metal semiconductor compound; and PA1 d) rapid thermal annealing (RTA) to convert, at least partially, the amorphous film into a polycrystalline film. The transition metal induces rapid crystallization of the amorphous film in a continuous, unidirectional, growth front. PA1 e) preheating the amorphous film at a temperature in the range between 400 and 500.degree. C.; and PA1 f) ramping the temperature up from the preheating temperature of step e) to the RTA temperature of step d) at a rate greater than 10.degree. C. per second, whereby lower temperature, transition metal-assisted crystal growth occurs during the incubation time. PA1 a) depositing a layer of the amorphous film having a first thickness; PA1 b) introducing a transition metal to the amorphous film; and PA1 c) rapid thermal annealing (RTA) to convert, at least partially, the amorphous film into a polycrystalline film. When the amorphous film is silicon, and the transition metal is nickel, step b) includes introducing the transition metal as ions implanted at a first dosage in the range between 1.times.10.sup.14 and 1.times.10.sup.16 ions/cm.sup.2.
When the amorphous film is silicon, and the transition metal nickel, the second thickness of nickel film is greater than 5 .ANG.. A thin nickel thickness induces crystallization of transistor active regions with low leakage currents. Further steps are included, before step d), of:
In forming a thin-film transistors with high electron mobility, a one-step annealing method for crystallizing an amorphous film is provided, comprising the steps of:
Also provided is a thin-film transistor (TFT) comprising a transparent substrate and a TFT polycrystalline semiconductor film, overlying the transparent substrate, formed from depositing an amorphous film on the substrate, and rapid thermal annealing the amorphous film with a transition metal. The TFT is fabricated through both the one and two step annealing methods, described above.